The Amazonian craton consists of the Guapore Shield in Brazil and the Guyana Shield in southern Venezuela, Guyana, Surinam, French Guiana and northern Brazil. It consists of several distinct geologic elements whose style of tectonism and age can be correlated with events in West Africa. Gneisses, amphibolites, itabirites and intrusive granites characterize the Imataca Complex in Venezuela. Pyroxene granulite facies rocks in the Imataca Complex have been dated between 3.6 and 2.7 Ga. Within Imataca Complex Gurian event was defined at about 3 Ga, which is equated to the Leonean in West Africa, and a later Imatacan event, dated at 2.7 Ga, is equivalent to the Liberian event in West Africa. The transamazonic event, dated at 2.0-1.7 Ga and characterized by felsic magmatism, correlates with the Eburnean event in West Africa. We can deduce that most of the Precambrian the Amazonian and the West African cratons were a single unit or in relatively close proximity.
Alluvial diamond deposits in Venezuela and Guyana are mainly associated with Roraima Formation, which suggest that sources are found within sedimentary sequences of it. All rivers and streams that flow along or across Roraima group contain diamonds. In Venezuela, terraces or recent gravels of Caroni, Paragua and Cuyuni Rivers and their tributaries have alluvial diamonds. In Guyana, Mazaruni, Potaro, Irang and Cuyuni Rivers were and are exploited for diamonds. Gravel of most of tributaries, such as Ekareku, Meamu, Kurupung, Eping, Arnik, Echirak of these major Rivers are diamond bearing.
To date, no kimberlite or lamproite indicator minerals, like pyrope garnet, Mg ilmenite, chromite or chrome diopside, have been found in heavy minerals suits in the alluvial deposits of Venezuela or Guyana. The only exception, in Venezuela, is alluvial diamonds deposit found in Quebrada Grande tributary of Rio Guaniamo. This deposit, worked since 1969, is covering an area of 15 km in length by 6 km wide and produced since it discovery over 6 million carats. Kimberlitic garnet, Mg ilmenite and chromite are found in heavy minerals of Guaniamo deposit.
Essentially all of the diamonds in the Guaniamo placer deposits are of local origin, and has been derived from kimberlite sills, rather than having been recycled from ancient sediments. Variations in characteristics of the diamond crystals from various deposits suggest that some have been derived from kimberlites that remain to be discovered.
The most probabl source of alluvial diamonds in Roraima Formation are layers of conglomerate found on the top of thick (800 m.) red sandstone layers. In Guyana, some crystals of diamonds found in alluvial deposits of Mazaruni and Kurubrang Rivers and its tributaries have thin brown to green outside layer (skin). This is characteristic of diamonds exposed to radiation for a long period of time. In conglomerate layers of Roraima Formation, associated with diamonds, uranium and thorium are abundant. As result, many alluvial diamond deposits in Guyana have higher than background thorium radiation levels.
The largest diamond found in 1942 in Venezuela at St. Elena, near Brazilian border, is 155 carats rough. The diamonds obtained from alluvial deposits in Venezuela and Guyana are mainly between 0.1 to 0.2 carats. Less than 5% of alluvial diamonds are above 0.5 carats. Only some 3 to 5 carats stones are found. For example, a miner's parcel of 800 carats of diamonds from Mazaruni River in Guyana had four stones between one and two carats and two stones of 3.2 and 4.8 carats each. The size of diamonds reduces with distance from Roraima Plateau gorges. Largest percentage of large diamonds is found in Rivers and Streams flowing over Roraima Formation, like Ekareku, Meamu, Kurubrang or Kurupung and tributaries. The gem to industrial ratio in Venezuela is 47% gem, 43% industrial and 10 bort, while in Guyana the ratio is 66% gem, 26% industrial and 8% bort.
In Venezuela the recorded production between 1950 and 1968 was in average 100,000 to 120,000 carats/year. The discovery of Guaniamo deposit increased the production of diamonds to 3.5 millions carats between 1970 and 1974. By 1979, 77% of diamonds produced in Venezuela came from Guaniamo deposit. The ratio of gem to industrial dropped to 27% gem 43 % industrial and 30% bort. In general paleo-alluvial deposits sources provides higher gem to industrial ratio than kimberlite or lamproite sources. This is a consequence of abrasion in a high-energy fluviatile transport. Diamonds with imperfections (flaws, fractures, etc) are reduced to diamond sand while diamonds without flaws are retained. In Mazaruni River some diamond bearing alluvials have up to 10 carats per 1 m3 of diamond sand.
In Guyana, the diamonds were mined from 1890 and the official production for 2003 is over 400,000 carats. This figure is from government records and represents an unknown percentage of actual production. The most significant fact is that in 2000 Guyana only produced around 80,000 carats, so the last 3 years jump in production reflects the contribution of Brazilian small miners that are recently moving into Guyana's diamond fields. It reminds me of January 1990, when first Brazilian crews arrived with their equipment to Guyana from Boa Vista as result of my promotional visit there. With brazilians arrived first portable diamond jigs and brazilian made gravel pumps, which greatly contributed to change for the better diamond mining techniques in Guyana. Previously, only sluice boxes were used to recover diamonds. My repeated test demonstrated that sluice box recovers only up to 5% of diamonds below 1 carat and 0% of larger stones. Still Guyanese small miners were making money using sluices as diamond recovery system. It only indicates how rich some of alluvial diamond deposits are. In 2004, at Itabali Landing on Mazaruni River, I met one of the first Brazilians that arrived with me in 1990. Now, he was the owner of two brand new CAT excavators... a positive change since 1990.
The average price for Guyanese diamonds is between US$100 to 120 per carat. The Venezuelan diamonds are of lesser quality and many visiting diamond dealers come to buy in Guyana, after visiting Venezuela and Roraima State in Brazil, to improve the quality and price of diamond parcels they will later sale in New York, London, Antwerp or Tel Aviv.
To be continued
Rafal Swiecki, geological engineer email contact
This document is in the public domain.